1. Field of the Invention
The present invention relates to a read circuit, and more particularly, to a read circuit for data signals recorded/reproduced in a disc player or the like.
2. Background of the Related Art
When a laser beam is scanned onto a disc such as CD, DVD or the like, a disc player converts reflected optical signals to electric signals using a photo sensor. When the photo sensor is divided into 4 sections, the disc player amplifies an output voltage in each of the sections by a preamp. The disc player inputs a radio frequency signal RFSUM in which the amplified signals are superimposed or compounded to a circuit for a radio frequency process. In the radio frequency process circuit, the output signal is restored through a DSP, a decoder, ect., to be displayed on a screen of a display apparatus or recovered as a sound by an audio player.
FIG. 1 is a block diagram showing a read circuit of a related art disc player. As shown in FIG. 1, the read circuit is composed of an automatic gain control (AGC) unit 1 outputting a radio frequency signal RFOUT at a predetermined level, an analog equalizer 2 receiving an output signal from the AGC unit 1 and compensating radio frequency attenuation effects and a 6 bit A/D converter 3 converting an output analog signal EQOUT from the equalizer 2 to a digital signal. A level compensating unit 4 obtains a mid value of the output signal EQOUT from the equalizer 2 and compares the value to a predetermined reference value VCM to adjust a level of a mid value of the output signal RFOUT from the AGC unit 1. A gain compensating unit 5 obtains and compares a peak value of the output signal EQOUT from the equalizer 2 to a predetermined reference value Vpeak to adjust a gain of the AGC unit 1. A timing control unit 6 detects a speed of an output signal Dout from the A/D converter 3 and controls a sampling time of the A/D converter 3.
The level compensating unit 4 is composed of an integral circuit 4-1, which includes a resistor R11 and a capacitor C11, that obtains a mid value of the output signal EQOUT of the equalizer 2 and a comparator AMP11. The comparator AMP11 compares an output signal from the integral circuit 4-1 and the reference value VCM. The comparator AMP11 adjusts a level of the mid value of the output signal RFOUT from the AGC unit 1 in accordance with the compared result.
The gain compensating unit 5 is composed of a peak value detector 5-1 detecting a peak value of the output signal EQOUT from the equalizer 2, a capacitor C12 and a comparator AMP12. The comparator AMP12 adjusts an amplification gain of the AGC unit 1 by comparing an output value from the peak value detector 5-1 to the reference peak value VPEAK and the capacitor C12 stabilizes an output from the comparator AMP12.
The timing control unit 6 is composed of a timing detector 6-1, a low-pass filter 6-2 and a voltage control oscillator 6-3. The timing detector 6-1 detects a frequency of the output signal Dout from the A/D converter 3 and outputs a control signal PU/PD in accordance with the result. The low-pass filter 6-2 converts the output signal PU/PD from the timing detector 6-1 to a direct current voltage. The voltage control oscillator 6-3 is controlled by the direct current voltage from the low-pass filter 6-2 to transmit a sampling clock signal SCLK to the A/D converter 3.
The operation of related art read circuit for the disc player will now be described. First, the radio frequency signal RFSUM is amplified in the AGC unit 1 and outputted as the signal RFOUT at a predetermined level. The output signal RFOUT from the AGC unit 1 is inputted to the equalizer 2 and also inputted to a servo block (not shown) for tracking and focusing control. The equalizer 2 receives the output signal RFOUT and compensates the frequency change. The A/D converter 3 converts the compensated signal EQOUT from the equalizer 2 to a digital signal Dout. As shown in FIG. 2, the equalizer 2 is an analog circuit configured filter with a gain characteristic that is flat in a low frequency range and boosted in a particular frequency range. The gain characteristic is boosted in the particular frequency range to compensate radio frequency effects or elements of 4 electric signals converted in the pickup unit that are attenuated when inputted to the circuit. The frequency and boost characters of the equalizer 2 are controlled by an externally input coefficient data K.
In the level compensating unit 4, the integral circuit 4-1 is composed of the resistor R11 and the capacitor C11. When the integral circuit 4-1 detects the mid value of the output signal EQOUT from the equalizer 2, the comparator AMP11 compares the detected mid value to the predetermined reference value VCM to output the comparison result to the AGC unit 1. When the reference value VCM is larger than the mid value of the output signal EQOUT from the equalizer 2, the mid value of the output signal RFOUT from the AGC unit 1 is increased. When the reference value VCM is smaller than the mid value of the output signal EQOUT, the mid value of the output signal RFOUT from the AGC unit 1 is decreased.
By having the mid value of the output signal EQOUT from the equalizer 2 become the reference value VCM, offsets of a number of amplifiers provided in the AGC unit 1 and the equalizer 2 are eliminated. The capacitor C11 is provided to maintain the stability of a negative feedback loop formed of the AGC unit 1, the equalizer 2 and the level compensating unit 4.
In the gain compensating unit 5, the peak value detector 5-1 detects a peak value of the output signal EQOUT from the equalizer 2. The comparator AMP12 compares the output signal EQOUT peak value to the predetermined peak value VPEAK and outputs the comparison result to the AGC unit 1.
When the reference value VPEAK is larger than the peak value of the output signal EQOUT from the equalizer 2, the gain compensating unit 5 increases the gain magnitude of the AGC unit 1. When the reference value VPEAK is smaller than the peak value of the output signal EQOUT, the gain compensating unit 5 decreases the gain magnitude of the AGC unit 1. Accordingly, the AGC unit 1 amplifies the radio frequency signal RFSUM and outputs the signal RFOUT at a constant level.
The capacitor C12 connected with an output terminal of the comparator AMP12 and has a capacity of a few .mu.F. The capacitor C12 is used to maintain the stability of the negative feedback loop of the AGC unit 1, the equalizer 2 and the gain compensating unit 5.
In the timing control unit 6, the timing detector 6-1, the low-pass filter. 6-2 and the voltage control oscillator 6-3 constitute a PLL loop to supply a sampling clock SCLK to the A/D converter 3. The timing detector 6-1 of the timing control unit 6 determines whether a phase of the output signal Dout of the A/D converter 3 is rapid or slow so that the A/D converter 3 may perform a sampling operation with an accurate sampling time at the peak value of the output signal EQOUT of the analog equalizer 2. Thus, the timing detector 6-1 outputs the control signal PU/PD to control the phase of the output signal Dout of the A/D converter 3. The low-pass filter 6-2 outputs a voltage, which is proportional to the signal PU/PD, to the voltage control oscillator 6-3. Thus, the voltage control oscillator 6-3, which is controlled by the voltage supplied from the low-pass filter 6-2, controls the speed of an oscillator frequency and maintains a constant frequency, for example 240 MHz. Thus, the timing control unit 6 modulates a sampling frequency and a phase of the A/D converter 3 to obtain the accurate sampling timing.
As described above, the related art read circuit has various problems. In the related art read circuit of the disc player, the analog equalizer 2 consumes a great deal of power and the power consumption increases as data recovery speed increases, which makes embodying a theoretical characteristic of the analog equalizer 2 difficult. For example, currently over 1W of electric power is consumed to achieve 3x of data recovery speed. Thus, the analog equalizer is difficult to use in a data recovery speed of 4x.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.